The present invention relates to a system for controlling an electromagnetic clutch for an automotive engine, and more particularly to a system for controlling clutch current at the engagement of the clutch.
An electromagnetic powder clutch as a type of electromagnetic clutch for a motor vehicle is known. The electromagnetic powder clutch comprises an annular drive member secured to a crankshaft of an engine, a magnetizing coil provided in the drive member, a driven member secured to an input shaft of a transmission and provided adjacent to the drive member defining a small gap therebetween and magnetic powder provided in a chamber in the clutch. Change speed gears in the transmission are changed by operating a selector lever. The selector lever is provided with a switch for the circuit of the magnetizing coil. When the selector lever is gripped by the operator's hand, the switch is opened to cut off the clutch current. Accordingly, change gear operation of the transmission can be done. When the selector lever is shifted to the gear engaging position, and released from the hand, the switch is closed so that electric current flows through the magnetizing coil to magnetize the drive member. As engine speed increases in accordance with the depression of an accelerator pedal of the vehicle, the clutch current applied to the coil increases with an increase of repetition frequency of ignition pulses. The magnetic powder is aggregated in the gap between the drive member and the driven member, so that the driven member is coupled to the drive member. More particularly, the clutch current passing through the magnetizing coil progressively increases according to the increase of frequency of the ignition pulses, while the clutch slips between the drive member and the driven member and gradually engages when the clutch current increases to a rated current. Thus, the motor vehicle may be smoothly started by depressing the accelerator pedal without operating a clutch pedal.
FIG. 1 shows the relationship between engine speed and clutch torque (clutch current). In a normal engine operation, the clutch torque varies as shown by a curve A. When the accelerator pedal is depressed, the clutch torque begins to increase at normal idling speed B. If a choke valve of the engine is closed or an air conditioning device of the vehicle is operated, the idling speed is increased to a high idling speed C and the clutch torque for starting the vehicle is increased to a large value at the high idling speed C. Accordingly, the vehicle starts abruptly by the large clutch torque.
A clutch current control system which is capable of eliminating the above described disadvantage has been proposed. The control system is adapted to slowly increase the clutch current in response to a choke signal or air conditioning signal. Thus, the clutch torque increases as shown by the dashed line in FIG. 1, so that the clutch may slip in a wider range, thereby smoothly starting a motor vehicle. However, the engine idling speed provided in response to the choke or air conditioning signal is not always constant. If the motor vehicle is started at a low idling speed, the clutch slips for a long time, which causes a low starting characteristic of the vehicle. If the vehicle is started at a high idling speed, which is caused, for example, by the effect of a dash pot device, the vehicle is abruptly started with a shock.